The present invention relates to a fuel injection valve for use in a direct-injection type internal combustion engine.
In a conventional diesel engine, there is a problem that while the fuel injection rate, i.e., injection quantity per unit time should be set at a suitably high value for improvement of the output characteristic of the engine as well as prevention of the emission of nitrogen oxides and other detrimental products, an increased fuel injection rate can lead to a correspondingly shortened injection period, which can cause a reduction in the combustion duration, resulting in a combustion noise, and can also cause an increase in maximum pressure within the engine cylinders.
On the other hand, in a conventional fuel injection valve used in such conventional diesel engine, the nozzle needle is urged by a single spring, so that the valve operation is monotonously affected by pressure changes within the injection pipe connected to the valve, that is, a specific amount of increase in the pressure within the injection pipe causes the same amount of increase in the lift of the nozzle needle in a lower injection quantity range as in a higher injection quantity range. Therefore, in a lower injection quantity range, the nozzle needle can lift through an excessive stroke to provide an excessive injection quantity due to an increase in the pressure within the injection pipe, and the resulting pressure drop in the injection pipe causes an undesirably small injection quantity during the next injection period, followed by an excessive injection quantity during the subsequent injection period. In this manner, the fuel injection valve suffers an unstable or irregular injection. Although a conventional fuel injection valve using a pintle nozzle can overcome such phenomenon of unstable or irregular injection due to the shape of its nozzle needle which produces a throttling effect, one using a hole nozzle is unable to avoid the above-mentioned phenomenon.
To solve this problem, a fuel injection valve has been proposed by the assignee of the present application in Japanese Utility Model Publication No. 54-3224, which includes a second nozzle spring arranged within its nozzle holder in addition to a conventionally employed first nozzle spring. In this proposed fuel injection valve, fuel injection takes place in two steps, i.e., an initial injection and a main injection, in such a manner that during the initial injection the first nozzle spring operates wherein the nozzle needle lifts through a limited stroke, and the main injection subsequently takes place with the valve opening pressure determined by the combined force of the first and second nozzle springs. This double-step injection reduces the injection rate throughout the whole injection period, thus substantially overcoming all the aforementioned drawbacks of combustion noise, emission of nitrogen oxides and irregular injection.
According to the proposed fuel injection valve, the valve opening pressure available at the initial injection is determined by the setting load of the first nozzle spring which takes part in the initial injection, and one available at the main injection by the sum of the setting loads of the first and second nozzle springs both taking part in the main injection, respectively. However, this fuel injection valve has the structural disadvantage that in adjusting the setting load of the second nozzle spring, it is impossible to bring the two nozzle springs into positions both seated on the injection hole portion through the nozzle needle, in which positions the two springs have their setting loads combined together, resulting in difficulties in achieving accurate setting of the setting load of the second nozzle spring so as to obtain a desired main injection valve opening pressure.
Further, the above conventional fuel injection valve is constructed such that adjustment of the lift of the nozzle needle for the initial injection has to be made by selecting the thickness of a shim interposed between a common spring seat for the two nozzle springs and a portion of the nozzle holder associated therewith, which makes it impossible to adjust the setting loads of the two nozzle springs independently of each other.